1. Field of the Invention
This invention relates to a semiconductor device and more specifically to, a power semiconductor device.
2. Related Background Art
Electronic devices such as personal computers and communication devices have power sources incorporating DC-DC converters, for example. For years, electronic devices are getting progressively small-sized, the drive voltage is getting lower, and the drive current is getting larger. Responsively, the need of a power source capable of efficiently supplying a large current and dealing with high frequencies has arisen.
To supply a large current under a low voltage, it is desirable that the ON resistance of the power semiconductor element used in the power source is as low as possible. To cope with high frequencies, the switching speed of the power semiconductor element used in the power source must be high.
For years, Schottky diodes are generally used in power sources for the purpose of rectification. Recently, power MOSFET has come to be used in lieu of Schottky diodes to enable the supply of a large current under a low voltage. Therefore, rectifying power MOSFET for rectification is required in addition to switching power MOSFET for switching the input and the output of the power source. This kind of power source is called a synchronous rectification circuit type power source because of synchronous switching motions of the rectifying power MOSFET and the switching power MOSFET.
FIG. 24 is a circuit diagram of a DC-DC converter 2000 used in a power source of a typical synchronous rectification circuit type. It is desirable that both the rectifying power MOSFET 2010 and the switching power MOSFET 2020 are switchable at a high speed because they are synchronously operated. In addition, both the rectifying power MOSFET 2010 and the switching power MOSFET 2020 preferably have the ON resistance as low as possible because a large current flows through MOSFET 2010 and 2020. Therefore, it is desirable to improve DC-DC converter 2000 of the synchronous rectification circuit type by decreasing the ON resistance of the switching power MOSFET 2020 and the rectifying power MOSFET 2010, increasing their switching speeds, and so on.
Some DC-DC converters include an inductance 2050 connected between the source electrode 2031 of the switching power MOSFET 2020 and the output 2040 of the DC-DC converter, for example. Switching of the power source including the inductance connected thereto is generally called L-load switching.
In the ON state of the switching power MOSFET 2020, the potential difference between the drain electrode 2060 and the source electrode 2031 is nearly zero, and electric energy is accumulated in the inductance 2050.
On the other hand, when the switching power MOSFET 2020 is switched from ON to OFF, connection between the drain electrode 2060 and the source electrode 2031 is interrupted. In addition, since the inductance 2050 behaves to maintain the current having got in the ON state of the switching power MOSFET 2020, the potential of the source electrode 2031 decreases. As a result, the voltage of the drain electrode 2060 is substantially clamped, and the potential difference between the drain electrode 2060 and the source electrode 2031 becomes larger than the potential difference between the input 2070 and the output 2040 of the DC-DC converter 2000. Depending on the magnitude of the inductance, the voltage between the drain electrode 2060 and the source electrode 2031 may exceed the withstanding voltage between the drain electrode 2060 and the source electrode 2031. In this case, an avalanche current by avalanche breakdown flows between the drain electrode 2060 and the source electrode 2031.
FIG. 25A is an enlarged cross-sectional view of a conventional switching power MOSFET 2020. The switching power MOSFET 2020 is symmetrical with respect to the broken line in FIGS. 25A. Therefore, explanation will be continued remarking the left side of the broken line.
The switching power MOSFET 2020 has the configuration explained below. A p−-type silicon layer 2105 is formed on a p++-type silicon substrate 2100. The drain electrode 2060 is connected to an n-type drain layer 2110 formed in the silicon layer 2105. An n+-type source layer 2140 is formed for connection from the n-type drain layer 2110 via a channel portion 2130 under the gate electrode 2080 to the source electrode 2030. The source electrode 2030 is connected also to a p-type base layer 2150 formed around the n+-type source layer 2140. A p+-type connecting layer 2160 is formed to connect the source electrode 2030 to the p++-type silicon substrate 2100. Further formed on the bottom surface of the silicon substrate 2100 is a source electrode 2031. Since the p+-type connecting layer 2160 extends to the silicon substrate 2100, the source electrode 2030 and the source electrode 2031 are electrically connected. As a result, a large current can flow when the channel portion 2130 is ON.
The channel portion 2130 is made up of the p−-type silicon layer 2105 and the p-type base layer 2150. Therefore, the n-type drain layer 2110, channel portion 2130 and n+-type source layer 2140 parasitically make up an npn bipolar transistor.
FIG. 25B is a circuit diagram of a parasitic npn bipolar transistor made up of the n-type drain layer 2110, channel portion 2130 and n+-type source layer 2140. The base of the parasitic npn bipolar transistor is connected to the source electrode 2030 via the p-type base layer 2150.
As already explained with reference to FIG. 24, an avalanche current by avalanche breakdown may flow between the drain electrode 2060 and the source electrode 2031. The avalanche current flows to the source electrode 2031 through the resistor of the p-type base layer 2150 and further through the p+-type connecting layer 2160 and p++-type semiconductor substrate 2100. If the current is large, a voltage drop occurring in the p-type base layer 2150 forwardly biases the junction between the n+-type source layer 2140 and the p-type base layer 2150. As a result, the parasitic npn bipolar transistor shown in FIG. 25B undesirably turns ON. Once the parasitic npn bipolar transistor turns ON, a still larger current flows between the drain electrode 2060 and the source electrode 2031. It results in raising the problem that the power MOSFET shown in FIG. 25A breaks (this phenomenon is hereinafter called “device breakdown by L-load switching”).
Still referring to FIG. 25A, the p+-type connecting layer 2160 must diffuse to reach the silicon substrate 2100. The p+-type connecting layer 2160 is diffused both vertically and laterally. If the p+-type connecting layer 2160 reaches the channel portion by lateral diffusion, the threshold voltage of the switching power MOSFET 2020 rises. Once the threshold voltage of the switching power MOSFET 2020 rises, its switching delays, and the ON resistance of the switching power MOSFET 2020 increases.
On the other hand, if the p+-type connecting layer 2160 is located apart from the channel portion not to reach the channel portion even upon lateral diffusion thereof, then the width of the switching power MOSFET 2020 increases and results in increasing the area of the DC-DC converter 2000. In case the switching power MOSFET 2020 is formed to occupy a constant area, it results in decreasing the unit cells of the switching power MOSFETs 2020 or the device units that can be made. Therefore, since the total channel width of the switching power MOSFETs 2020 within a given area becomes narrower, the ON-state current decreases. It results in substantially increasing the ON resistance of the switching power MOSFET 2020.
FIG. 26 is an enlarged cross-sectional view of another model of conventional switching power MOSFET 2020. FIG. 25 shows a MOSFET 2020 having a flat-type gate, but the FIG. 26 shows a MOSFET 2020′ having a vertical-type trench gate.
The MOSFET 2020′ includes a source layer 2220 connected to a source electrode 2210, a drain layer 2240 connected to a drain electrode 2230 and a trench gate electrode 2260 buried between the source layer 2220 and the drain layer 2240 via an insulating film 2250.
This MOSFET 2020′ efficiently uses the side surface of the gate electrode, and thereby reduces the ON resistance.
However, since the trench gate electrode 2260 is adjacent to the drain layer 2240 via the insulating film 2250, and the insulating film 2250 is thinned to lower the ON resistance, the MOSFET 2020′ involves the problem that the parasitic capacitance between the trench gate electrode 2260 and the drain layer 2240 becomes large. Because of this parasitic capacitance, the MOSFET 2020′ is slow in switching, and it is not suitable for use as a high-frequency switch.
The rectifying power MOSFET 2010 also involves the same problems discussed above.